What is a takeoff slot in aviation?

Aviation slots

A slot system is brought in to control the number of aircraft in the air, and around the runway at an airport Slots are applied at airports

slot 911 A departure slot is a time when an aircraft can take off, the so called Calculated Time of Take-Off

slot deposit termurah Airport slots regularly appear in aviation news, especially regarding busy airports, but what are they, how do they work, and why are they